Solid-state rechargeable battery

ABSTRACT

A solid-state rechargeable battery includes a pair of electrode layers and a solid electrolyte layer interposed between the pair of electrode layers. The pair of electrode layers each include a phosphate having an olivine crystal structure. The phosphate contains a transition metal and lithium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Application No.2015-212665, filed Oct. 29, 2015, in the Japanese Patent Office. Alldisclosures of the document(s) named above are incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state rechargeable batteryincluding a solid electrolyte.

2. Description of the Related Art

Lithium ion rechargeable batteries and electric double-layer capacitorshave been developed actively as large-capacity electrochemical devicesand are starting to be used in various applications such as consumerappliances, industrial machinery, and automobiles. In rechargeablebatteries including an electrolyte solution, for example, a leakage ofthe electrolyte solution may occur. Accordingly, solid-state batteries,which are constituted by solid components only, have been developed byusing a solid electrolyte. The solid-state batteries commonly include apositive current collector, a positive electrode layer, a solidelectrolyte layer, a negative electrode layer, and a negative currentcollector.

There have been disclosed methods in which a multilayer body constitutedby thin layers is used in order to enhance the responsivity and capacitydensity of a solid-state lithium ion rechargeable battery. For example,Japanese Patent No. 5122154 discloses a method for producing amultilayer solid-state battery. It is intended to enhance the energydensity of the solid-state battery by employing a multilayer structure.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to increase thedischarge capacity of a solid-state rechargeable battery that has beencharged and the operating potential of the solid-state rechargeablebattery at which the solid-state rechargeable battery is discharged inorder to further increase the use of the solid-state rechargeablebattery.

A solid-state rechargeable battery according to an aspect of the presentinvention includes a pair of electrode layers and a solid electrodelayer interposed between the pair of electrode layers. The pair ofelectrode layers each include a phosphate having an olivine crystalstructure. The phosphate contains a transition metal and lithium. Thetransition metals contained in the phosphates included in the electrodelayers may be the same as or different from each other and arepreferably the same as each other. It is more preferable that thephosphates included in the electrode layers have the same chemicalcomposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a solid-state rechargeablebattery according to an embodiment of the present invention.

FIGS. 2A to 2E are schematic cross-sectional views of a multilayer body,illustrating an example method for producing the multilayer body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is described below with referenceto the attached drawings. Note that the present invention is not limitedto the embodiment illustrated in the drawings. In the drawings, thefeatures of a solid-state rechargeable battery according to theembodiment may be emphasized; all components of the solid-staterechargeable battery are not always illustrated on an exact scale in thedrawings.

FIG. 1 is a cross-sectional view of a solid-state rechargeable batteryaccording to an embodiment. The solid-state rechargeable battery has abasic structure including a pair of electrode layers 1 b and 2 b and asolid electrolyte layer 3 interposed therebetween. The electrode layers1 b and 2 b include current collectors 1 a and 2 a, respectively,through which electrons are released and received. In general, one ofthe pair of electrode layers serves as a positive electrode and theother electrode layer serves as a negative electrode depending on, forexample, the difference in potential between the electrodes.

In this embodiment, the electrode layers 1 b and 2 b each include aphosphate having an olivine crystal structure which contains atransition metal and lithium. In other words, both electrode layer 1 band electrode layer 2 b include such a phosphate. Such a phosphate hasbeen commonly used as a positive electrode active material. An olivinecrystal structure is the structure of crystals of natural olivine. Thepresence of an olivine crystal structure can be determined by X-raydiffractometry.

A typical example of a phosphate having an olivine crystal structurewhich contains a transition metal and lithium is LiCoPO₄, which containsCo as a transition metal. Other phosphates denoted by the above chemicalformula in which the transition metal has been changed may also be used.In such a case, the proportions of Li and PO₄ vary in accordance withthe valence number of the transition metal. Preferable examples of thetransition metal include Co, Mn, Fe, and Ni.

When a phosphate having an olivine crystal structure which contains atransition metal and lithium is included in an electrode layer thatserves as a positive electrode, it serves as a positive electrode activematerial as in the related art. When a phosphate having an olivinecrystal structure which contains a transition metal and lithium isincluded in an electrode layer that serves as a negative electrode, itincreases the discharge capacity of the battery and the operatingpotential of the battery at which the battery is discharged. Althoughthe mechanisms by which such a phosphate produces the above advantageouseffects are not fully clarified, this is presumably because thephosphate partially forms a solid solution together with a negativeelectrode active material.

The phosphate included in each of the pair of electrode layers 1 b and 2b contains a transition metal, which may be the same as or differentfrom that contained in the other phosphate. In other words, thephosphates included in the electrode layers 1 b and 2 b may contain thesame transition metal or different transition metals. The electrodelayers 1 b and 2 b may each contain only one transition metal or two ormore transition metals. It is preferable that the electrode layers 1 band 2 b contain the same transition metal. It is more preferable thatthe phosphates included in the electrode layers 1 b and 2 b have thesame chemical composition. When the electrode layers 1 b and 2 b containthe same transition metal or include a phosphate having the samecomposition, the similarity between the compositions of the electrodelayers 1 b and 2 b is increased. In such a case, even if the solid-staterechargeable battery according to the embodiment is unintentionallyconnected with reversed polarity, the solid-state rechargeable batterymay endure practical use without causing a malfunction depending on theuse of the battery.

One of the pair of electrode layers 1 b and 2 b may further include asubstance commonly used as a negative electrode active material. Addinga negative electrode active material to only one of the electrode layersmakes it clear that the electrode layer serves as a negative electrodeand the other electrode layer serves as a positive electrode. It is alsopossible to add a substance commonly used as a negative electrode activematerial to both of the electrode layers. Examples of the negativeelectrode active material include compounds such as titanium oxide,lithium-titanium composite oxide, carbon, and lithium vanadiumphosphate, which are used in the related art regarding rechargeablebatteries.

In the production of the pair of electrode layers 1 b and 2 b, forexample, solid electrolyte materials and conducting materials (i.e.,conductant agents) such as carbon and metals may further be used inaddition to the above active materials. These materials may be uniformlydispersed in water or an organic solvent together with a binder and aplasticizer to form an electrode-layer-forming paste. Examples of themetals used as a conductant agent include Pd, Ni, Cu, Fe, and alloys ofthese metals.

Examples of a conducting metal constituting the current collectorsconnected to the respective electrode layers 1 b and 2 b include, butare not limited to, single-element metals such as Ni, Cu, Pd, Ag, Pt,Au, Al, and Fe, alloys of these metals, and oxides of these metals. Theelectrode layers and current collectors are formed using the aboveelectrode-layer-forming paste and a current-collector-forming conductingmetal paste. For example, printing may be performed on a green sheet ofa solid electrolyte layer described below by using theelectrode-layer-forming paste, and printing is again performed using theconducting metal paste. The printing method is not limited, and variousprinting methods known in the related art, such as screen printing,intaglio printing, letterpress printing, and calendar rolling, may beemployed. Although it is considered to be the most common to use screenprinting for producing a highly multilayer device constituted by thinlayers, it may be suitable to use ink-jet printing in some cases whereelectrodes having a fine pattern or a special shape need to be formed.

In this embodiment, the electrode layers 1 b and 2 b may have the samecomposition. In such a case, the battery is symmetrical in terms ofpolarity. This eliminates the need for paying attention to the polarityof the battery when the battery is connected.

In this embodiment, the solid electrolyte layer 3 is composed of asubstance that is solid at normal temperature. Various substances knownin the related art may be used as a material of the solid electrolytelayer. It is preferable to use, for example, a phosphate having aNASICON structure which contains lithium. Such a phosphate is known inthe related art as a material of solid electrolytes and may be usedwithout particular limitations. A typical example of such a phosphate iscomposite lithium phosphate containing Ti. It is possible to add a metalelement such as Al, Ge, Sn, Hf, Zr, Y, or La to the composite lithiumphosphate. Al may be replaced with another trivalent transition metalsuch as Ga, In, or La. Specific examples of a phosphate having a NASICONstructure which contains lithium include, but are not limited to,Li—Al—Ge—PO₄ materials and LiTi₂(PO₄)₃. It is preferable to use aLi—Al—Ge—PO₄ material that contains the same transition metal as thatcontained in the phosphates included in the electrode layers 1 b and 2b. For example, in the case where the electrode layers 1 b and 2 binclude a phosphate containing Co and Li, it is preferable that thesolid electrolyte layer contain a Li—Al—Ge—PO₄ material containing Co.This reduces the elution of the transition metal contained in theelectrode active materials to the electrolyte.

A method for forming the solid electrolyte layer is not limited, andvarious methods known in the related art may be used appropriately. Forexample, the above phosphate material is prepared so as to have anappropriate grain size distribution and uniformly dispersed in anaqueous solvent or an organic solvent together with a binder, adispersant, a plasticizer, and the like to form a slurry. In thepreparation of the slurry, a bead mill, a wet jet mill, various types ofkneaders, a high-pressure homogenizer, and the like may be used. Inparticular, it is preferable to use a bead mill, with which the controlof the grain size distribution of the phosphate material and thedispersion of the phosphate material can be performed at the same time.The resulting slurry is applied to a film in order to form a green sheethaving a predetermined thickness. The coating method is not limited, andvarious coating methods known in the related art may be used. Examplesof the coating methods include, but are not limited to, a slot diemethod, reverse coating, gravure coating, bar coating, and a doctorblade method.

A method for producing a multilayer body is not limited, and productionmethods known in the related art may be used appropriately. Commonly,precursors (e.g., green sheets) of the pair of electrode layers and thesolid electrolyte layer are stacked on top of one another. Subsequently,a print layer composed of the conducting metal paste which serves as aprecursor of the current collector is formed on the stacked precursors.The stacked precursors are press-bonded to one another by a suitablemethod to form a multilayer body (i.e., precursor of a multilayerportion). This multilayer body is subsequently fired. Firing of themultilayer body may be performed in an oxidizing atmosphere or anonoxidizing atmosphere. The maximum temperature in the firing step ispreferably set to, but not limited to, 400° C. to 1000° C. and is morepreferably set to 500° C. to 900° C. A step in which the multilayer bodyis maintained at a temperature lower than the maximum temperature in anoxidizing atmosphere may be conducted in order to remove the binder to asufficient degree before the maximum temperature is reached. It isdesirable to perform firing at a low temperature in order to reduce theprocess cost. A reoxidation treatment may optionally be performedsubsequent to the firing step. The solid-state rechargeable batteryaccording to an embodiment of the present invention is produced in theabove-described manner.

FIGS. 2A to 2E are schematic cross-sectional views of the multilayerbody, illustrating an example method for producing the multilayer body.A precursor 10 b of an electrode layer is prepared as illustrated inFIG. 2A. A print layer composed of a conducting metal paste which servesas a precursor 10 a of a current collector is formed on one surface ofthe precursor 10 b as illustrated in FIG. 2B. Subsequently, anotherprecursor 10 b is formed on the surface of the precursor 10 a such thata multilayer structure including two precursors 10 b of electrode layersand the precursor 10 a of a current collector interposed therebetween isformed as illustrated in FIG. 2C. A plurality of the multilayer bodiesillustrated in FIG. 2C are prepared in the above-described manner andstacked on top of one another. When the multilayer bodies are stacked ontop of one another, precursors 31 to 35 of solid electrolyte layers maybe interposed between each pair of the multilayer bodies as illustratedin FIG. 2D. Subsequently, the stacked precursors are press-bonded to oneanother to form a multilayer body illustrated in FIG. 2E. The multilayerbody is fired in the above-described manner and may optionally be cutinto a desired shape.

EXAMPLES

An embodiment of the present invention is described more specificallywith reference to Examples below. However, the present invention is notlimited to the embodiment described in Examples below.

Example 1 Preparation of Raw Materials

Raw material compounds (Li₂CO₃, Al₂O₃, GeO₂, NH₄H₂PO₄, Co₃O₄) were mixedsuch that the molar ratio between Li₂O, Al₂O₃, GeO₂, P₂O₅, and Co₃O₄ was24.38/5.63/11.25/56.25/2.50. The resulting mixture was fired in the airat 850° C. to synthesize a solid electrolyte. An XRD analysis of thesolid electrolyte confirmed that the solid electrolyte mainly had aNASICON crystal structure.

Raw material compounds were mixed such that the molar ratio betweenLi₂O, Co₃O₄, and P₂O₅ was 37.50/25.00/37.50 and the resulting mixturewas fired to synthesize a positive electrode active material. An XRDanalysis of the positive electrode active material confirmed that thepositive electrode active material was composed of only the LiCoPO₄phase having an olivine crystal structure. Raw material compounds(Li₂CO₃, Al₂O₃, TiO₂, NH₄H₂PO₄, and Co₃O₄) were mixed such that themolar ratio between Li₂O, Al₂O₃, TiO₂, P₂O₅, and Co₃O₄ was24.38/5.63/11.25/56.25/2.50. The resulting mixture was fired in the airat 850° C. to synthesize a negative electrode active material. An XRDanalysis of the negative electrode active material confirmed that thenegative electrode active material mainly had a NASICON (sodium (Na)Super Ionic Conductor) crystal structure.

Preparation of Sheets

The synthesized positive electrode active material, the synthesizednegative electrode active material, and metal Pd were mixed in anethanol-toluene mixed solvent such that the volume fractions of thepositive electrode active material, the negative electrode activematerial, and metal Pd were 23.3%, 46.7%, and 30.0%, respectively. Aplasticizer and a binder were added to the resulting mixture, and themixture was subsequently kneaded with a bead mill. The kneaded mixturewas applied to a PET film in order to prepare a green sheet of anelectrode layer. The synthesized solid electrolyte was also formed intoa slurry and applied to a film as in the preparation of a green sheet ofan electrode layer. Thus, a green sheet of a solid electrolyte wasprepared.

Preparation of Multilayer Body

A Pd paste was applied to the green sheet of the electrode layer byscreen printing in order to form a Pd current collector. Two greensheets of an electrode layer on which the Pd current collector wasformed, 2 green sheets of an electrode layer on which the Pd currentcollector was not formed, and 30 green sheets of a solid electrolytewere each cut into a piece having a diameter of 16 mm by punching. ThePd current collector layer, the electrode layer, 30 solid electrolytelayers, the electrode layer, and the Pd current collector layer werestacked in this order such that the Pd current collector layers disposedon the respective sides of the multilayer body faced outward. Thestacked layers were press-bonded to one another at a pressure of 30 MPato form a plate-like body. The plate-like body was interposed betweenhigh-purity alumina plates and fired at a time in an N2 atmosphere at600° C. to form a sintered plate.

Evaluation 1

The sintered plate including electrode layers disposed on the respectivesurfaces, which was prepared by firing, was encased in a 2032 coin cellin a glove box filled with an argon atmosphere. Thus, an evaluationsolid-state battery cell was prepared. This cell was subjected to acharge-discharge test at 150° C. Specifically, the cell was charged at acurrent of 36 μA until a voltage of 2.7 V was reached and thendischarged at a current of 18 μA until a voltage of 2 V was reached. Theinitial discharge capacity of the cell was 152 μAh. The operatingpotential at the time an amount of electric charge which corresponded to50% of the discharge capacity of the cell was discharged was 2.42 V.

Evaluation 2

One of the current collector layers and one of the electrode layers wereremoved by grinding one surface of the sintered plate. An SPE film and ametal lithium foil were formed on the surface of the sintered plate fromwhich the current collector layer and the electrode layer were removed.The resulting sintered plate was encased in a coin cell as inEvaluation 1. Thus, an evaluation sample was prepared. Thecharge-discharge characteristics of the remaining electrode layer thatserved as a positive electrode were evaluated. Specifically, the cellwas charged at a current of 36 μA until a voltage of 5.1 V was reachedand discharged at a current of 18 μA until a voltage of 4 V was reached.Two stages of plateau in the vicinity of 4.7 to 4.9 V, which aredistinctive characteristics of LiCoPO₄, were observed during bothcharging and discharging of the cell. The initial discharge capacity ofthe cell was 142 μAh. The term “plateau” is a battery term that refersto a region in which the potential is flat.

Evaluation 3

An evaluation sample was prepared as in Evaluation 2, and thecharge-discharge characteristics of the remaining electrode layer thatserved as a negative electrode were evaluated. Specifically, adsorptionof Li was performed at a current of 36 μA until a voltage of 2 V wasreached, and desorption of Li was performed at a current of 18 μA untila voltage of 3 V was reached. The charge-discharge potential was 2.3 to2.4 V, which was slightly lower than that of a NASICON-type Li—Al—Ti—P—Ocompound, that is, 2.5 V. It was confirmed that the potential variedcontinuously compared with a NASICON-type Li—Al—Ti—P—O compound. This ispresumably because of LiCoPO₄ that was present abundantly in thenegative electrode. The initial Li desorption capacity of the negativeelectrode was 240 μAh. The initial coulombic efficiency was 95%. Theoperating potential at the time an amount of Li ions which correspondedto 50% of the Li desorption capacity was desorbed was 2.42 V.

Comparative Example 1

A solid-state battery was prepared as in Example 1, except that apositive electrode was prepared from the green sheet of the electrodelayer used in Example 1 and the green sheet described below was preparedand used as a negative electrode. The charge-discharge characteristicsof the solid-state battery were evaluated as in Evaluation 1 above. Thegreen sheet of the negative electrode was prepared such that theelectrode layer did not contain LiCoPO₄, that is, the volume fractionsof a NASICON-type Li—Al—Ti—P—O compound and Pd were 70% and 30%,respectively. The above-described battery was charged at a current of 36μA until a voltage of 2.7 V was reached and discharged at a current of18 μA until a voltage of 2 V was reached. The initial discharge capacitywas 72 μAh. The potential at the time an amount of electric charge whichcorresponded to 50% of the total discharge capacity was discharged was2.25 V. A comparison between the results of Example 1 and the results ofComparative Example 1 confirmed that adding LiCoPO₄ to anegative-electrode-side electrode layer enhanced the operating potentialof the battery.

Comparative Example 2

An evaluation sample was prepared as in Evaluation 3 above, except thatthe green sheet of an electrode layer was prepared so as not to containLiCoPO₄, that is, such that the volume fractions of a NASICON-typeLi—Al—Ti—P—O compound and Pd were 70% and 30%, respectively. Thechare-discharge characteristics of the evaluation sample were evaluated.Specifically, the adsorption of Li was performed at a current of 84 μA,until a voltage of 2 V was reached, and the desorption of Li wasperformed at a current of 42 μA until a voltage of 3 V was reached. Theinitial Li desorption capacity of the negative electrode was 80 μAh. Theinitial coulombic efficiency was 40%. The operating potential at thetime an amount of Li ions which corresponded to 50% of the Li desorptioncapacity was desorbed was 2.58 V. A comparison between the results ofEvaluation 3 in Example 1 and the results of Comparative Example 2confirmed that the absence of LiCoPO₄ increased the operating potentialof the negative electrode and the presence of LiCoPO₄ increased theavailable voltage.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

We claim:
 1. A solid-state rechargeable battery comprising: a pair ofelectrode layers each including a phosphate containing a transitionmetal and lithium, and having an olivine crystal structure; and a solidelectrolyte layer interposed between the pair of electrode layers. 2.The solid-state rechargeable battery according to claim 1, wherein thephosphate included in each of the pair of electrode layers contains thesame transition metal.
 3. The solid-state rechargeable battery accordingto claim 2, wherein the transition metal comprises at least one elementselected from Co, Mn, Fe, and Ni.
 4. The solid-state rechargeablebattery according to claim 2, wherein the solid electrolyte layercontains a transition metal the same as that contained in the phosphatesincluded in the electrode layers.
 5. The solid-state rechargeablebattery according to claim 1, wherein one of the electrode layersfurther comprises a NASICON structure.
 6. A method for producing asolid-state rechargeable battery, the method comprising: preparing afirst precursor of a first electrode layer; forming a print layercomposed of a conducting metal paste on one surface of the firstprecursor; forming a second precursor of a second electrode layer onanother surface of the print layer opposite to that formed on the onesurface of the first precursor; forming a multilayer structure includingthe first and second precursors of the first and second electrode layersand the print layer interposed therebetween; preparing a plurality ofthe multilayer structures and stacking on top of one another; formingthird precursors of solid electrolyte layers interposed between eachpair of the multilayer structures; press-bonding the stacked precursors;and firing the stacked precursors.